Underground pipes such as sewer pipes, water-supply pipes, cable protective pipes, etc., are installed using, for example, a thrust boring method. The thrust boring method, as disclosed in Japanese Laid-Open Patent Publication No. 58-120996, is such that a vertical hole is bored into the ground from the wall of which a pipe having a drilling cutter at the forward end thereof is pushed into the ground to be thrust through it in the horizontal direction for installation of the pipe while the drilling cutter is boring a horizontal tunnel of a diameter slightly larger than that of the pipe. To the rear end of the pipe pushed into the ground, a new pipe to be installed is connected and pushed into the ground to be thrust through the horizontal tunnel.
The thrust boring method disclosed in the above Japanese Publication uses a pipe having a collar on the end thereof facing opposite to the thrusting direction, the collar being used to connect the next pipe.
In the case of the underground pipe disclosed in the above Japanese Publication, since the collar has a larger diameter than that of the pipe body excluding the collar, a large gap is created between the horizontal tunnel and the outer surface of the pipe. Therefore, the soil in the tunnel may fall and accumulate on the bottom of the tunnel while the pipe is being thrust therethrough. If the soil accumulates on the bottom of the tunnel, the collar of the pipe that is pushed into the tunnel thereafter may override the soil, causing the thrusting direction of the pipe to turn upward and resulting in a deviation from the desired direction. This may also result in the bending of the pipe being thrust. Also, since a gap of the size equivalent to the difference between the outer diameter of the collar and the outer diameter of the pipe is left around the pipe installed underground, the ground may sink by the depth equivalent to the size of the gap if the ground is not firm enough. In the case of using a pipe joint to join the pipes being thrust through the tunnel, if the pipe joint has a larger diameter than that of the pipe, the same problem as mentioned above will occur.
As a solution to such a problem, in the case of a thick wall pipe such as a Hume pipe, the outer circumferential surface of the end portion of the pipe on which the collar is to be fitted may be ground down to sufficient depth so as not to allow the collar fitted thereon to protrude outwardly beyond the outer surface of the pipe body. However, in the case of a plastic pipe with a thin wall thickness such as a PVC pipe, the outer circumferential surface of the end portion of the pipe on which the collar is to be fitted can only be ground down to a maximum of 40% of its wall thickness if the strength of the end portion of the pipe on which the collar is fitted is to be retained. Therefore, the thickness of the collar to be fitted on the ground down end portion of the pipe should be, at maximum, approximately 40% of the wall thickness of the pipe if the collar is not allowed to protrude beyond the outer circumferential surface of the pipe body. If the collar is also made of synthetic resin like the pipe, the collar cannot be made sufficiently strong with this thickness, and may break when the pipe is thrust into the ground. It can be considered to provide a collar made of metal, or other material having excellent stiffness, with a separate construction from that of the pipe. However, when such a metal collar is fitted onto the pipe made of synthetic resin, it is extremely difficult to quickly bond them together for sufficient water tightness, and therefore, it is not possible to employ such a construction for the underground pipe for the thrust boring method.
In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide an underground pipe for a thrust boring method and a connecting construction of the underground pipe for the thrust boring method, wherein there is no possibility of the thrusting direction being altered or the pipe being bent because of the buildup of soil on the bottom of a horizontal tunnel when the pipe is thrust through the tunnel, and also, the pipe itself is sufficiently strong so that no breakage will occur.